This is a level 1 data flow diagram of a library (book) information system. The example illustrates the main processes within the system, the kind of information that flow inside and outside of it, where the data gets stored, and how the basic processes convert the information from one form to another. This level of Data Flow Diagram limits its focus only to the system of interest. As you can see on the example below the external entities that are connected to the system and evoke the actions within the system are not displayed.